To explain it a little bit, basically what I'm doing is multiplying the color in the first image by the color in the second image. Even if you're not a programmer, you can see that not a lot of code is required.

The two inputs to the GLSLShader module (the Image module and the Video module) are multiplied together, resulting in the alpha mask effect. (I put the Starfield in the background just so you can see where the video has become transparent.)

Note that circle.png and shortCars.avi are both in the Magic sample projects folder, so you should be able to duplicate the scene exactly. Or, try it out with different inputs to the GLSLShader module and I'm sure you'll have a lot of fun playing around with it.